(1) Field of the Invention
The present invention relates to organic electroluminescence display devices and, more particularly, to a technique for sealing an organic electroluminescence display device in order to prevent degradation of an organic EL material which is brought about by moisture.
(2) Description of the Related Art
Hitherto, of display devices, CRT display devices were dominant. However, in lieu of the CRT display devices, flat display devices such as liquid crystal display devices, plasma display devices and the like have come in practice, and the demand has arisen for the flat display devices. Moreover, in addition to these display devices, the development and practical application of a display device utilizing an organic electroluminescence technology (hereinafter referred to as “organic EL display device”) and a display device in which electron sources utilizing a field emission technology are arranged in matrix form and phosphors arranged at a positive electrode are made bright, whereby an image is produced (hereinafter referred to as “FED display device”) have progressed.
The organic EL display device has the following characteristics. (1) The organic EL display device does not need backlights since it is of a spontaneous light type as compared to a liquid crystal display device; (2) in the organic EL display device, voltage required for light emission is low, namely, 10 V or less, and there is a chance of being able to reduce power consumption; (3) as compared to the plasma display device and an FED display device, the organic EL display device does not need a vacuum structure and is suitable for weight-lightening and thickness reduction; (4) the response time of the organic EL display device is short, namely, several microseconds, and the animation characteristic of the organic EL display device is excellent; (5) the visual field angle of the organic EL display device is wide, namely, 170 degrees or more; and the like.
However, if moisture and/or oxygen is present around an organic EL material, the oxidation of the material speeds up, dark spots are generated, and the luminescence characteristic is deteriorated. In order to address this problem, after a wiring, a switching element, an organic EL layer and the like are provided on a substrate, a glass substrate or a sealing can for sealing is mounted to a back face of the substrate, to thereby hermetically seal an interior of a display device, and a desiccant member is disposed within the interior of the display device, whereby moisture is eliminated from the interior of the display device within which the organic EL material is provided.
Referring now to FIGS. 14 and 15, conventional display devices will be discussed hereinafter in order to facilitate understanding of the present invention. FIG. 14 is a schematic sectional view of a conventional example in which sealing of the display device is performed utilizing a sealing can 7. Though an undercoat layer, a wiring, a switching element, and the like are provided on a substrate 1, they are have been left out of the illustration. An organic EL film 3 emits light by applying voltage between a lower electrode 2 and an upper electrode 4. The sealing can 7 is mounted to the substrate by a sealing member 8, whereby an interior of the display device is kept hermetically sealed. In order to eliminate moisture from the interior, a desiccant member 5 is mounted inside the sealing can 7. The desiccant member 5 is fixed inside the sealing can 7 by a double-sided adhesive tape 6. As a material for the sealing can 7, metal such as stainless steel and the like is employed. As a material for the desiccant member 5, active carbon, zeolite, silica gel, or the like is employed. Moreover, in order to improve a color balance of emitted light colors, a polarizing plate 11 is stuck onto an outer surface of the substrate.
FIG. 15 is a schematic sectional view of another example of the conventional display device, in which the interior of the display device is kept hermetically sealed by a rear glass plate 9. The rear glass plate 9 is mounted through a sealing frame 10 to a substrate 1 by sealing members 8 in order that a space between the rear glass plate 9 and the substrate 1 is kept, whereby the interior of the display device is kept airtight. In this case, a desiccant member 5 is fixed on an inner surface of the rear glass plate 9 by a double-sided adhesive tape 6. As a material for the desiccant member 5 of FIG. 15, the same material as the material is employed for the desiccant member mounted to the sealing can 7 of FIG. 14 is employed. In order to improve a color balance of emitted light colors, bonding of a polarizing plate 11 onto an outer surface of the substrate 1 is carried out in the same manner as the polarizing plate is bonded onto the outer surface of the substrate 1 in the example of FIG. 14.
The above-mentioned conventional examples are disclosed in Japanese Patent Application Laid-Open Publication Nos. H3-261091 and 2001-345175, for example.
In the organic EL display devices shown in FIGS. 14 and 15, the polarizing plate is bonded to the substrate in order to improve the color balance of the emitted light colors. At the time of the bonding of the polarizing plate, such force as to be indicated by a designator F in FIGS. 14 and 15 is applied to the sealing can or the rear glass plate from the back due to repulsive force. Moreover, when air bubbles are generated between the substrate and the polarizing plate bonded onto the substrate, the display device is placed in a pressure vessel in order to eliminate the air bubbles, that is, the display device is subjected to the so-called autoclave processing. At this time, the same force is also applied to the rear glass plate or the sealing can.
When such force is applied to the sealing can or the rear glass plate, they are made to flex inward. When they are made to flex inward, if a gap g (shown in FIGS. 14 and 15) between the desiccant member and the upper electrode or the organic EL layer is small, the desiccant member and the upper electrode or the organic EL layer make contact with each other, whereby the organic EL layer is broken. A thickness of the upper electrode is 150 nm, while a thickness of the desiccant member is about 0.15 mm. Moreover, the organic EL layer has, for example, a five-lamination-layer structure, but the total thickness of five layers is just about 130 nm. Thus, when the desiccant member and the upper electrode or the organic EL layer make contact with each other, the organic EL layer is easily destroyed.
Moreover, if a screen of the display device is made large-sized, an amount of the above-mentioned deflection of the sealing can or the rear glass plate becomes larger, so that the foregoing problem becomes more serious. One of the characteristics of the organic EL display device is to be able to make the thickness of the entire display device thin. However, if the gap g shown in FIGS. 14 and 15 is made large in order that the contact between the desiccant member and the upper electrode or the organic EL layer can be avoided, the advantage of the organic EL display device is lost. On the other hand, even if the thickness of the sealing can or the rear glass plate is made large in order that the deflection of the sealing can or the rear glass plate is made reduced, the display device not only becomes thick as a whole but also becomes heavy.